1. Field of the Invention
The present invention relates to a thermal image-recording apparatus which comprises a thermal print head.
2. Description of the Prior Art
In the thermal printing process, a dye-bearing donor ribbon is brought into contact with a dye-receiving print sheet at a print zone. Thermal printing is effected by contacting the donor ribbon with a multi-element print head which spans the ribbon in a direction transverse to the direction of ribbon travel. The print head typically comprises a linear array of closely spaced resistive heating elements, each being independently addressable by an applied voltage to heat that portion of the donor ribbon directly opposite and thereby cause dye to transfer from the ribbon to the print sheet.
The print sheet is wrapped about the surface of a rotatable print drum. Only the leading edge of this sheet is clamped on the surface of the drum (leading: i.e. considered according to the direction of rotation of the drum), whereas angular wrapping as such of the sheet occurs spontaneously as the sheet is sandwiched between the drum surface and the donor ribbon at the print zone.
The printing process described hereinbefore can be used for producing opaque as well as transparent prints. The former are prints on white or coloured paper that are intended for direct reading, whereas the latter can be so-called overhead projection prints for optical projection on a screen, or transparencies for medical diagnosis on a light box, e.g. prints of echographic (ultrasound) or NMR (nuclear magnetic resonance) examinations of a patient.
The donor ribbon is in the form of a web-type dye-carrier containing a series of spaced frames of different coloured heat-transferable dyes, wound on a supply spool. The ribbon is paid out from the supply spool and rewound on a take-up spool. The dye ribbon is difficult to handle and very vulnerable since it has typically a thickness in the order of magnitude of 8 to 10 micrometers only, in order not to impede the heat transfer from the heating elements towards the print sheet. For that reason, the supply spool and the take-up spool are usually provided in a dedicated cassette which has a large central opening allowing the print head to urge the ribbon in contact with the print sheet.
It has been shown that the donor ribbon can become damaged by contact with the trailing edge of the print sheet prior to the dye transfer, viz. in the zone of the central opening of the cassette extending between the supply spool and the print head. Said edge being not attached to the print drum, the trailing end of the sheet moves away from the drum under the influence of the inherent stiffness of the sheet.
The problem is particularly harmful in colour printing since in such case the print drum makes four revolutions for printing in succession a yellow, a magenta, a cyan and possibly a black separation image, and each revolution of the drum involves the risk of damage of the corresponding frame of the dye ribbon.
Arrangement of a thermal printer to have the leading as well as the trailing end of a print sheet attached to the drum would seriously complicate the construction of the machine.